1. Field of the Invention
This invention relates to a gel glass technique for making a germanium-containing silicate glass for optical fibers and devices.
2. Description of the Prior Art
Monolithic gel oxide glass preparation has recently received increased interest primarily because of the striking differences in the fabrication of bulk glass when compared with conventional techniques, and associated potential advantages resulting from these differences, such as high homogeneity and purity. Gel glasses are formed from liquid precursors which undergo a solidification through a slow, continuous increase in viscosity. Theoretically dense glass is formed without exceeding temperatures more than 200.degree. C. above the glass transition. From the initial through the final stages of fabrication, the material remains transparent, although it generally changes density by approximately a factor of 2 and changes bulk volume by approximately 80 percent. An exciting prospect is that multicomponent compositions which were difficult to fabricate by conventional techniques because of significantly different thermal characteristics of the individual oxide precursors can now be mixed to high homogeneity and formed to solid glass at relatively low temperatures.
Of particular interest are glasses in the binary GeO.sub.2 --SiO.sub.2 system because of their importance to lightguide fiber and devices. Germania (GeO.sub.2) is known to increase the index of refraction of silica glass. The cores of silica optical fibers typically include germania, which helps confine optical energy to the core when surrounded by a lower index of refraction cladding. Similarly, optical waveguides formed on a substrate typically obtain guiding of optical energy by forming a relatively higher index of refraction region surrounded by a lower index of refraction region. Other optical functions, including polarizers, diffraction gratings, directional couplers, delay lines, lenses, etc., make use of regions of higher refractive index material in silica. In nonoptical applications, glasses are used as passivating layers; for example, in semiconductor device fabrication, germanium is known to typically lower the temperatures at which a silicate glass will flow.
Most gel work done today is based on using a silicon alkoxide as the primary constituent; see, for example, Treatise On Materials Science And Technology, S. Sakka, Vol. 22, pages 129-167 (1982). Silicon alkoxides hydrolyze slowly and are easily formed into monolithic gels. Other alkoxides have been added to silicon alkoxide to form multicomponent monolithic gels, but appear to result in nonoptical quality glass; see "GeO.sub.2 /SiO.sub.2 -Glasses from Gels to Increase the Oxidation Resistance of Porous Silicon Containing Ceramics," J. Schlichting and S. Neumann, Journal of Noncrystalline Solids, Vol. 28, pages 185-193 (1982). Glasses containing limited amounts of GeO.sub.2 have been prepared using a colloidal gel, which can be prepared by hydrolysis to form a translucent solid; see U.S. Pat. No. 3,954,431, coassigned with the present invention. The rate of hydrolysis is typically too rapid to yield a monolith. While this was a suitable method of making glasses, the microstructure of the resultant material is such that it has to be heated to temperatures well above the sublimation points of the dopants in order to obtain a clear bulk glass. Typically the gel was fired at a low temperature, ground to a fine powder, and subsequently fused by injection into an rf plasma flame. As the concentration of dopant is increased, the degree of inhomogeneity resulting from sublimation is also increased. It is desirable to obtain germanium dopant levels which exceed the limits of the earlier technique used to make homogeneous glasses.